Ventilation increases as arterial pO2 decreases.
Elastance is the reciprocal of compliance.
Higher PaCO2 levels result in higher ventilation rates for the same arterial pO2.
The cerebral cortex.
Lung compliance is best described using pressure–volume curves of the lungs under static conditions.
PCO2, PO2, and pH in arterial blood.
The volume of air remaining in the lungs after a normal expiratory breath.
Under static conditions, transpulmonary pressure reflects the elastic recoil pressure of the lungs.
In neonatal respiratory distress syndrome, lung compliance is greatly reduced due to insufficient surfactant.
Dorsal respiratory group (DRG), Pneumotaxic area, and Ventral respiratory group (VRG).
The amount of nitrogen at the start is the same amount that ultimately is distributed between the lung and the bag, allowing calculation of RV or FRC.
Mechanical work of breathing = Force × Distance = Pressure × Volume. Thus, the work of breathing = cumulative product of pressure × volume of air moved over time = ΔP × ΔV/Δt.
The volume of air remaining in the lungs after maximal expiration.
PCO2 and pH.
In hypothetical saline-filled lungs, compliance is greatly increased as the lack of an air–fluid interface means that no surface tension exists.
Located in the medulla, it controls inspiration.
La Paz, Bolivia, at 3630 m (11,910 ft).
It evaluates gas exchange by providing information about oxygenation, acid–base balance, chronicity, and severity of respiratory failure.
Total lung capacity (TLC), residual volume (RV), and functional residual capacity (FRC).
The occurrence of AMS depends on the elevation, the rate of ascent, and individual susceptibility.
Forced vital capacity (FVC), forced expiratory volume in one second (FEV1), and the ratio of these two volumes (FEV1/FVC).
Hyperventilation lowers arterial PaCO2, which increases alveolar pressure of oxygen.
A restrictive ventilatory defect, such as pulmonary fibrosis or kyphoscoliosis.
Hyperventilation results in hypocarbia, reducing the normal stimulus to ascend (hypercarbia), leaving hypoxia as the only stimulus. This can lead to severe hypoxaemia, hypoxic seizures, or loss of consciousness on ascent.
Bradycardia, vasoconstriction (redistribution of blood flow to myocardium, lungs, and brain), and splenic contraction.
It gets released and is viewed as 'useful' work.
The total volume of air in the lungs after maximal inspiration.
The work required to overcome the elastic recoil of the chest wall and lungs, airway resistance, and viscosity during inspiration.
Static compliance is the lung compliance obtained during ‘static’ conditions when there is no gas flow activity within the lungs. It monitors only elastic resistance and can be used to select the ideal level of PEEP during mechanical ventilation.
Areas of the lung will be perfused but not ventilated, resulting in an increase in V/Q mismatch.
The patient's basal metabolic rate drops by 15% following induction of anaesthesia due to thalamic inhibition.
The barometric pressure is 6.25 kPa, meaning inspired PiO2 is zero.
Hypoxic stimulation of the peripheral chemoreceptors located in the aortic and carotid bodies.
To provide a simple method to measure airways obstruction, particularly in asthmatic patients.
CSF alkalosis, which is transient as bicarbonate is excreted from the CSF over 24-48 hours and renally excreted.
400–600 mL.
Mild AMS can be treated with analgesics for headache (e.g., ibuprofen), acetazolamide, and dexamethasone.
At PaO2 < 8 kPa.
The ventilatory response to arterial pO2.
Lung compliance (CL) is calculated as ΔV/ΔP.
Boyle’s law, which states that at a constant temperature, within a closed system, pressure is inversely proportional to volume.
It may increase to 2–4 ml O2/L/min.
The subject rebreathes from a bag of known volume containing nitrogen-free gas, and the nitrogen concentration decreases as it equilibrates with the gas in the bag.
Located in the medulla, it regulates expiration.
The number of oxygen molecules per breath is reduced.
PaCO2 will increase, leading to an increase in hydrogen ion concentration and a fall in pH, which is detected by central chemoreceptors, resulting in an increased respiratory rate.
The loop shows a scooped-out pattern during expiration, indicating reduced airflow.
An obstructive ventilatory defect, such as asthma or COPD.
Surface tension in alveolar fluid provides a force that tries to collapse the alveolus. Lung surfactant reduces surface tension, increasing lung compliance and making the alveolus less likely to collapse. Lack of surfactant, as seen in neonatal respiratory distress syndrome, decreases compliance.
By sampling end-expiratory gas for carbon monoxide after a patient inspires a small amount of CO, holds their breath, and exhales.
1200–1500 mL.
Nifedipine, a calcium channel blocker, reduces pulmonary artery pressure by inhibiting hypoxic pulmonary vasoconstriction, improving oxygen transfer, and can be used to treat HAPO.
2000 mL.
Increasing arterial partial pressure of oxygen via increased barometric pressure, which increases dissolved oxygen, improves oxygen diffusion, promotes angiogenesis, improves polymorph function, inhibits anaerobe growth, and displaces carbon monoxide from hemoglobin.
They live in high-pressure chambers to keep their bodies saturated in nitrogen and decompress slowly at the end of their diving period.
Approximately 65%.
Approximately 0.5–1.0 ml O2/L/min.
The phrenic nerve and intercostal nerves.
They provide voluntary control over breathing.
They measure only communicating gas, unlike body plethysmography which can measure both communicating and non-communicating gas.
It represents the relationship between the flow rate of air and the volume of air during inspiration and expiration.
Barometric pressure increases by 1 atm for every 10 m descent.
HAPO results from increased pulmonary extravascular lung water, preventing effective oxygen exchange. Symptoms include shortness of breath at rest, chest tightness, persistent cough with white or frothy fluid, marked fatigue, feeling of suffocation at night, confusion, and irrational behavior.
654 L/min.
Helium/oxygen gas mixtures are used because helium does not exhibit the narcotic properties of nitrogen.
Increased erythropoietin secretion results in a slow increase in red cell count to increase oxygen-carrying capacity, which also raises haematocrit and can lead to thrombosis.
Nitrogen absorbed into body tissues forms bubbles during rapid ascent, causing microvascular complications.
To allow their mucociliary and inflammatory cell function to return to somewhere approaching normal.
Hypercarbia augments the ventilatory response to hypoxia.
They measure partial pressure and assume sea level atmospheric pressure (101 kPa). At high altitude, they may under-read the percentage of oxygen.
Because peripheral chemoreceptors respond to the partial pressure of oxygen rather than the oxygen content of the blood.
Gas will follow the path of least resistance into the non-dependent lung (the uppermost lung).
Extracellular fluid.
Lung compliance is the measure of distensibility, defined as the change in lung volume (ΔV) per unit change in transpulmonary pressure (ΔP).
Through the limbic system and hypothalamus.
They include stretch receptors, joint, and muscle proprioceptors that help regulate breathing.
They require more advanced techniques such as gas dilution or body plethysmography.
To identify patients at high risk of perioperative pulmonary complications and reduce these risks through patient preparation, targeted anaesthetic, surgical techniques, and planning for appropriate post-operative care.
From higher CNS structures, peripheral and central chemoreceptors, and mechanoreceptors in the lungs and chest wall.
The barometric pressure is about 64 kPa (480 mmHg), resulting in roughly 40% fewer oxygen molecules per breath.
The timed measurement of dynamic lung volumes during forced expiration and inspiration.
4800 mL.
The area and thickness of the blood-gas barrier, the volume of blood in the pulmonary capillaries, and the distribution of alveolar volume and ventilation.
By measuring spirometry before and after the administration of a bronchodilator. An improvement in FEV1 of 200 ml or more, or an improvement of >15% if the baseline FEV1 is >1.5 l, infers significant reversibility.
It stimulates respiration but at high levels causes narcosis, increases cerebral blood flow, and intracranial pressure.
Baroreceptors are mechanoreceptors that respond to stretch, also known as stretch or pressure receptors. They are terminal myelinated nerve endings located within vessel walls and the cardiac chambers.
The changes described are exaggerated in those patients with lung disease.
3000 mL.
There is an increase in capillary density, reducing oxygen diffusion distance, and a change in intracellular oxidative enzymes favoring cellular respiration under hypoxic conditions.
Normal resting breath volume.
The rate of discharge increases, leading to a reduction in sympathetic outflow and an increase in parasympathetic transmission, which reduces blood vessel tone, heart rate, and contractility, thereby lowering blood pressure.
By stimulating the inspiratory center to increase respiratory rate.
Probably via stagnant hypoxia.
Because CSF has less protein than blood.
Residual volume (RV), Total lung capacity (TLC), and Functional residual capacity (FRC).
FRC represents the pulmonary oxygen store.
They are involved in the automatic control of breathing.
Total wasted energy due to tissue and airway losses.
Static compliance is measured under ‘static’ conditions when there is no gas flow, such as during an inspiratory pause. The subject breathes into a spirometer to measure lung volumes, and an oesophageal pressure probe is used to estimate intrapleural pressures. The compliance is calculated from the gradient of the pressure–volume curve.
By measurement of maximum mouth pressures.
Both central and peripheral chemoreceptors detect the rise in PaCO2 and fall in pH.
The slope of the P-V loop is steepest around FRC but reduces at both low and high lung volumes. FRC is affected by factors such as age, body posture, and body size, which in turn affect lung compliance.
The VRG neurons are stimulated and drive the expiratory muscles.
433 L/min.
In spontaneous, awake ventilation, expiration is a passive movement; under anaesthesia, it becomes an active one.
1200 mL.
As ml/min/mmHg and as a percentage of a predicted value.
Information on airflow, O2 consumption, CO2 production, and heart rate, which is used to compute other variables such as oxygen uptake and the anaerobic threshold.
Furosemide, a loop diuretic, may be used to treat pulmonary oedema acutely but can lead to collapse from low-volume shock if the victim is dehydrated.
Helium is 50% less soluble than nitrogen, so less dissolves into tissues, reducing the risk.
Many days.
Gas will be drawn into the dependent lung (the lowermost lung).
Volume of air that can be expired from end of normal tidal volume.
40% in the dependent lung and 60% in the non-dependent lung.
The slope of the pressure–volume curve equates to lung compliance.
To maintain homeostasis of pH, PaO2, and PaCO2 in the blood.
FRC can provide a 10-minute oxygen store during apnoea.
Supine position, general anaesthesia, pregnancy, and obesity.
History of pre-existing lung disease, smoking history, exercise tolerance, respiratory symptoms, number and frequency of hospital admissions with respiratory problems, and current treatment regimen.
AMS is a common condition at high altitude, occurring in 75% of people over 3000 m (10,000 ft). Symptoms include headache, nausea, dizziness, loss of appetite, fatigue, shortness of breath, disturbed sleep, and general malaise.
RV (Residual Volume) and TLC (Total Lung Capacity).
They reduce the ventilatory response to CO2.
Systemic vasodilatation, myocardial depression, and arrhythmias.
Rapid ascent can cause pressure differences that may lead to pneumothoraces or perforated tympanic membranes.
The respiratory quotient (R) represents CO2 production divided by O2 consumption, typically 0.8.
Through bicarbonate retention and urinary hydrogen ion excretion.
Increase in heart rate and stroke volume from sympathetic stimulation due to hypoxia, leading to an overall rise in myocardial work.
Recompression, which forces nitrogen back into solution.
The maximum volume expired after a maximal inspiration.
Gas lesions (air or gas emboli, decompression sickness), infections (refractory osteomyelitis, necrotizing soft tissue infections, clostridial infections), global hypoxia (carbon monoxide poisoning, severe anemia), and regional hypoxia (compromised grafts or free flaps, osteoradionecrosis, crush injuries).
Glomus cells, which contain dopamine.
6.5 kPa and 4.5 kPa.
Specific compliance is compliance divided by FRC (Functional Residual Capacity), compensating for differing body sizes.
Normal lung compliance is 200 mL/cm H2O.
A reflex triggered by stretch receptors in the lung to prevent over-inflation.
Standing position, COPD, asthma, and PEEP.
Both methods measure only communicating gas and use the same principle for determining RV or FRC.
Spirometry is the standard method for measuring most relative lung volumes.
Expiration is passive.
They would succumb to hypoxia and lose consciousness.
The loop is smaller and shifted to the right, indicating reduced lung volumes.
The loop shows a plateau in both inspiration and expiration, indicating a limitation in airflow.
It increases pulmonary vascular resistance.
General anaesthesia can cause atelectasis and inhibit hypoxic pulmonary vasoconstriction, potentially leading to increased V/Q mismatching.
Baroreceptors alter their action potential firing rate in response to changes in blood pressure, creating a negative feedback mechanism responsible for the autonomic regulation of blood pressure.
In the aortic bodies (near the aortic arch) and carotid bodies (bifurcation of the common carotid artery).
A few hours.
Volume of air that can be inspired over and above the resting tidal volume.
Because they rely on neural transmission, which is extremely fast, allowing them to mediate rapid changes in blood pressure, such as the bradycardia observed after administration of a vasopressor like phenylephrine.
Approximately 35%.
In the brainstem, composed of nuclei within the medulla and pons.
Hysteresis is an important phenomenon seen in P-V curves; it represents ‘unrecoverable’ energy because the lungs do not act as a perfect elastic system. At any given lung volume, the pressure required to inflate the lung is greater than that required for deflation.
It facilitates gas exchange between the alveoli and the blood.
The subject sits in an airtight chamber, inhales or exhales to a particular volume, and makes respiratory efforts against a closed shutter, causing changes in chest and box volume and pressure.
6000 mL.
The ability of gas to transfer from alveoli to red blood cells across the alveolar epithelium and the capillary endothelium.
There is a right shift in the oxyhaemoglobin dissociation curve caused by increased levels of 2,3-DPG, favoring oxygen unloading.
Applying PEEP can help 'splint' the alveoli open but may also reduce blood flow to the splinted areas and destabilise the cardiovascular system in particularly sick patients.
Dexamethasone is a corticosteroid with anti-inflammatory properties, useful in reducing cerebral oedema.
140 meters.
Intubation decreases dead space, but this effect is reduced by connectors and other equipment.
Low-pressure baroreceptors are located in the chambers of the heart, large systemic veins, and the pulmonary vasculature. They bring about changes in blood volume and are involved in the slower and sustained control of blood pressure.
Hypoxia and the partial pressure of oxygen in arterial blood.
Their most important role is in response to a fall in blood pressure, such as during hemorrhage or when standing up.
45% in the dependent lung and 55% in the non-dependent lung.
FRC is dependent on the balance of the tendency of the lungs to recoil and the thoracic cage to expand.
Increasing tidal volume, increasing respiratory flow, and increasing airway resistance (e.g., COPD).
Located in the pons, it assists in regulating inspiration.
Dynamic compliance is the lung compliance obtained under ‘dynamic’ conditions when gas flow activity is present during rhythmic breathing. It monitors both elastic resistance and airway resistance.
Dynamic compliance is measured under ‘dynamic’ conditions when there is gas flow, such as during rhythmic breathing. The subject breathes into a spirometer to measure lung volumes, and an oesophageal probe is used to estimate intrapleural pressures. Compliance is typically calculated during a tidal breath at the points of zero flow on the P-V loop.
Upper abdominal surgery, thoracic surgery, and open vs. laparoscopic procedures.
Gas dilution tests and body plethysmography.
During maximum inspiratory effort against an occlusion at residual volume or at FRC.
Compression of gas-filled cavities such as the lungs, middle ear, and sinuses occurs.
Adjustments for alveolar volume (estimated from dilution of helium) and the patient’s haematocrit.
Nausea, tinnitus, twitching, and convulsions.
It is safe for a diver to rapidly halve their ambient pressure, e.g., from 10 m depth (2 atm) to the surface (1 atm).
Because the non-dependent lung's total compliance is greater as it does not have the weight of the thorax pressing down on it.
By preoxygenation (denitrogenation).
The highest permanent habitation is found in the Andes mountain range at 4877 m (16,000 ft) above sea level.
The closing capacity is the volume of the lungs at which the small airways begin to collapse and close off.
Age >70 years, history of lung disease, BMI >30, and smoking history >20-pack year.
It can terminate inspiration prematurely by sending inhibitory impulses, effectively 'fine-tuning' inspiration.
The response to hypercapnia is blunted, and the acute responses to acidosis and hypoxia are almost entirely abolished.
Based on clinical assessment and the nature of the planned surgery.
A period of acclimatisation during which physiological adaptation occurs in response to the relative lack of oxygen.
2500 mL.
There is an overall left shift in the oxyhaemoglobin dissociation curve, favoring oxygen uptake in the pulmonary capillaries.
High-pressure arterial baroreceptors and low-pressure baroreceptors.
100% oxygen reduces the effects of altitude sickness.
High-pressure baroreceptors in the aortic arch and carotid sinus discharge impulses along the vagus and glossopharyngeal nerves to the nucleus tractus solitarius in the medulla. This modulates sympathetic and parasympathetic outflow, restoring blood pressure towards normal.
They abolish the peripheral chemoreceptor response to hypoxia.
CSF compensation occurs via increased HCO3− transport into the CSF.
The expiratory work returned, which is passive under resting conditions and active during stress conditions.
Correlation of history, examination findings, and relevant investigation results in conjunction with the nature of proposed surgery.
They are mainly inspiratory neurons that control inspiration and are responsible for basic ventilatory rhythm.
Thoracic gas volume and airway resistance.
Symptoms usually start 12–24 hours after arrival at altitude and begin to decrease in severity around the third day.
Lung elasticity is due to elastin and collagen in lung tissue. Aging and conditions like emphysema, which involve loss of elastic tissue, increase lung compliance. Pulmonary fibrosis and pulmonary congestion reduce compliance due to increased collagen deposition.
PAO2 = PiO2 – (PACO2 / R), where PAO2 is alveolar partial pressure of oxygen, PiO2 is inspired pressure of oxygen, PACO2 is alveolar partial pressure of carbon dioxide, and R is the respiratory quotient.
At high barometric pressures, nitrogen has narcotic properties, limiting safe use of air to depths of 30-50 m.
A flow volume loop is constructed from spirometric data, with expiratory flow above the x-axis and inspiratory flow below the x-axis. It contains diagnostic information.
It is used to evaluate both cardiac and pulmonary functions, determine maximal exercise capacity, and identify which organ systems contribute to symptoms of exertional dyspnoea and exercise intolerance.
Hypoxaemia stimulates ventilation through its effects on the carotid and aortic bodies (peripheral chemoreceptors).
It results in an increase in pulmonary vascular resistance, which can lead to right heart failure.
Because they can provide specialized knowledge and expertise.
There is no effect on alveolar ventilation below a PaCO2 of 4 kPa.
Blood is preferentially distributed to the lower lung, while gas flows into the non-dependent lung.
Via the peripheral chemoreceptors.
For up to 48 hours.
Atmospheric pressure halves every 5500 m (18,000 ft).
It allows for easier interpretation of perioperative changes in gas exchange.
Factors affecting lung compliance include lung volume, lung elasticity, and surface tension. Lung volume affects the slope of the P-V loop, lung elasticity is influenced by elastin and collagen, and surface tension is affected by alveolar fluid and surfactant.
The force-generating capacity of inspiratory muscles.
Functional residual capacity (FRC) falls by 15–20% due to a loss of muscle tone.
Lung compliance is reduced, airway resistance increases slightly, and mucociliary transport mechanisms are reduced, increasing the work of breathing and causing retention of secretions.
HACO is a life-threatening condition resulting from brain tissue swelling due to fluid leakage. Symptoms include headache, weakness, disorientation, loss of coordination, decreasing levels of consciousness, loss of memory, hallucinations, psychotic behavior, and coma.
Acetazolamide is a carbonic anhydrase inhibitor that reduces bicarbonate formation, increases hydrogen ion concentration, and causes metabolic acidosis, leading to respiratory compensation and increased minute ventilation, thus lowering PaCO2.
Alveolar dead space rises from 0 to 70 ml, and physiological dead space increases from 150 to 220 ml.
Primary pulmonary hypertension, pulmonary embolism, emphysema, and pulmonary fibrosis.
Yes, at oxygen partial pressures greater than 2 atm, which equates to air diving at depths greater than 40 meters.
At least 12 hours, because the half-life of carbon monoxide is 4 hours.
2 liters per 100 grams of tissue per minute.
60% in the dependent lung and 40% in the non-dependent lung.
CO2 diffuses across the blood-brain barrier into the CSF, generating hydrogen ions and lowering pH, which stimulates the inspiratory area.
Increased barometric pressure causes pulmonary vascular pressure to exceed alveolar pressure, leading to pulmonary oedema and difficulty in breathing.
High-pressure arterial baroreceptors are located within the walls of the aortic arch and carotid sinus. They control perfusion pressures to the coronary and cerebral circulations and are involved in the rapid short-term control of blood pressure.
Cranial nerves X (vagus) and IX (glossopharyngeal).
Tec vaporisers function normally at altitude, delivering a constant partial pressure of volatile agent, not a constant volume percentage.
Untreated pneumothorax, gas trapping in the lungs (e.g., lung bullae, bronchospasm), and certain drugs (e.g., doxorubicin).
30% in the dependent lung and 70% in the non-dependent lung.
Polycythaemia and alveolar haemorrhage.
It results in an increase in alveolar ventilation of approximately 1–2 L/min.
The rate of discharge decreases, leading to increased sympathetic outflow, which helps to restore blood pressure.
pH changes.
In the ventral medulla.
